When you say "the delete feature" do you mean the remove library feature, at the top right of the library window, or the minus sign near the bottom which only becomes visible when you select an object.

If the former, just choose the add library feature (next to the delete feature) This will open a navigation window, go to the downloads folder, which is in the poser 11 content, select this (or the runtime folder in it, either will do), and click and it should be restored to your library window.